dt-bindings: power: Add Spreadtrum SC2731 charger documentation

Spreadtrum SC2731 PMIC battery charger binding

Required properties:

 - compatible: Should be "sprd,sc2731-charger".

 - reg: Address offset of charger register.

 - phys: Contains a phandle to the USB phy.

Optional Properties:

- monitored-battery: phandle of battery characteristics devicetree node.

  The charger uses the following battery properties:

- charge-term-current-microamp: current for charge termination phase.

- constant-charge-voltage-max-microvolt: maximum constant input voltage.

  See Documentation/devicetree/bindings/power/supply/battery.txt

Example:

	bat: battery {

		compatible = "simple-battery";

		charge-term-current-microamp = <120000>;

		constant-charge-voltage-max-microvolt = <4350000>;

		......

	};

	sc2731_pmic: pmic@0 {

		compatible = "sprd,sc2731";

		reg = <0>;

		spi-max-frequency = <26000000>;

		interrupts = <GIC_SPI 31 IRQ_TYPE_LEVEL_HIGH>;

		interrupt-controller;

		#interrupt-cells = <2>;

		#address-cells = <1>;

		#size-cells = <0>;

		charger@0 {

			compatible = "sprd,sc2731-charger";

			reg = <0x0>;

			phys = <&ssphy>;

			monitored-battery = <&bat>;

		};

	};